1. Field of the Invention
The present invention relates to a relay optical system used for photographing, with an electronic image-pickup camera or the like, an image formed by an objective lens.
2. Description of Related Art
As measures to record an image formed by an objective lens of a microscope, there are the photography using a silver halide camera and the photography using a TV camera. According to the photography with a silver halide camera, a sample image is recorded on a film.
On the other hand, the photography with a TV camera is disclosed in Japanese Patent Application Preliminary Publication (KOKAI) No. Hei 6-331903, where a TV camera connecting tube that comprises an ocular observation barrel, an adapter, an attachment for a TV camera, and a photographing section of the TV camera is shown. Although an image sensor is not particularly described in this document, a solid-state image sensor (CCD), for example, may be used.
Conventionally, the number of pixels of a solid-state image sensor is roughly determined in accordance with the number of scanning lines of a TV monitor. For example, an image sensor with 512×512 pixels or 640×512 pixels is used for the standard format (NTSC), and an image sensor with 1024×768 pixels is used for the high-definition type.
As described above, according to the conventional photography, a sample image is recorded on a film, while, in recent years, digital cameras using, as the recording medium, solid state image sensors have appeared in the market, to be popularized. The digital camera is characterized in its large number of pixels for the area of the solid-state image sensor. In early years, hundreds of thousands of pixels would be provided for the ⅓-inch type, while, in recent years, one or two millions of pixels or more are provided for the ⅔-inch or ½-inch type.
However, the digital camera is constructed to have a photographing lens fixed to the camera body and thus the entrance pupil position is placed inside the photographing lens or the camera body. Therefore, if a person would try to use the camera in combination with a microscope for photographing a sample image, he has to position the body of the digital camera close to the lens barrel of the microscope so as to make the exit pupil position (or a position conjugate with the exit pupil position) of the microscope coincide with the entrance pupil position of the digital camera. As a result, the microscope and the digital camera would exclude each other from their predetermined positions, which is a problem.
It is noted that each of Japanese Patent Application Preliminary Publication (KOKAI) No. Hei 2-222914, Japanese Patent Application Preliminary Publication (KOKAI) No. Hei 9-54258, Japanese Patent Application Preliminary Publication (KOKAI) No. Hei 9-133875 and Japanese Patent Application Preliminary Publication (KOKAI) No. Hei 10-39235 discloses an optical system used for observation of an image (intermediate image) formed by an objective lens. However, such an optical system is directed for observation via human eyes and thus is difficult of use as a relay optical system for a photographing apparatus.